Cryptococcus neoformans is a ubiquitous pathogenic fungus that causes disease in humans usually manifested as meningitis. In AIDS patients, crytococcosis is the fourth most commonly seen opportunistic infection and the most life-threatening fungal infection. Understanding of the interaction between C. neoformans and the host, which involves multiple arms of the immune response and perhaps multiple virulence factors of the organism, will allow for the development of strategies for protection. Clinical and experimental evidence indicate that cell mediated immunity (CMI) is an important host defense in cryptococcosis. Experimental infection with C. neoformans as well as injection with cryptococcal crude culture filtrate down-regulates anti-C. neoformans CMI. The inhibition of CMI has been explained by the generation of a cascade of cryptococcal- specific suppressor cells and soluble factors. The component responsible for the induction of suppression has not been determined. The capsule of C. neoformans is an important virulence factor, but experimental evidence suggests that other virulence factors are probably involved. The fact that virulence of C. neoformans is not a direct function of the capsular diameter and the lack of evidence for the in vivo role of the other virulence factors described, stimulated this proposal. We have detected inhibition of the phagocytic and immune response by cytoplasmic extracts of C. neoformans. Preliminary experiments suggest that this inhibition is probably not related to the capsular polysaccharide material. Impairment of the immune response seen in patients with cryptococcosis could be explained by the production of metabolites by C. neoformans that inhibit the phagocytic and immune responses. Purification of the component responsible for the in vitro immunosuppression will be accomplished by isoelectric focussing as well as gel and ion exchange chromatography. Purification will be assessed by polyacrylamide gel electrophoresis. A rat model, which mimic human cryptococcosis in its chronicity and central nervous system involvement, will allow us to test the immunosuppressive effect of the purified cytoplasmic fractions. For the evaluation of the anti-phagocytic activity a sensitive and quantitative assay will be used. The long term goals of this proposal are: 1 - to purify and characterize the immunosuppressive component detected in cytoplasmic extracts from C. neoformans; 2 - to determine the in vitro and in vivo relevance of this inhibitory component; 3 - to assess the role of this immunosuppressive component on the virulence of C. neoformans.